The invention relates to improvements in a quick-disconnect structure by which a load-handling attachment is removably mounted upon a lift truck carriage to permit quick mounting or demounting of the attachment. In particular, the invention relates to a vertically-slidable, upwardly-facing hook structure mounted adjacent the bottom of a load-handling attachment for engaging the downwardly-protruding lip of a standardized lower horizontal mounting bar on the front of a lift truck carriage.
Quick-disconnect hooks for engaging such standardized lower horizontal mounting bars are now commonly used on lift truck load-handling attachments. Most such hooks are rotatably mounted to the rearwardly-facing surface of the attachment frame so as to pivot about a forwardly-extending axis, such as the hooks shown in U.S. Pat. Nos. 4,230,434 and 4,406,575. Alternatively, other types of quick-disconnect hooks pivot about a transverse axis, such as that shown in U.S. Pat. No. 4,482,286 or, although not pivotable, utilize a transverse locking pin. A disadvantage of all of the foregoing hook assemblies is that the pivot axis pin or transverse locking pin, as the case may be, must absorb all, or at least part, of the reactive force preventing forward movement of the attachment relative to the bottom of the lift truck load carriage. Such reactive force can be very substantial incident to the pulling or withdrawing of a load, or the striking of an obstacle by the attachment or load as it is being lowered by the lift truck mast. The shear and bending stresses imposed on such pins by such reactive force frequently cause bending or breakage of the pins.
A few prior quick-disconnect hook assemblies feature either a pivotal or vertically-slidable hook locked in its engaged position by a transverse pin which is not required to absorb such reactive force. However these structures have a hook whose forwardly-facing engagement surface, when in its engaged position, is located below the upper extremity of the hook-mounting member. Such assemblies, if used on the rear surface of a load-handling attachment to engage the lower horizontal bar of a lift truck carriage, would require a space between the rear surface of the attachment and the forward surface of the lift truck carriage sufficient to accommodate the hook-mounting member. Any such space reduces the load-carrying capacity of a counterbalanced lift truck by increasing the forward protrusion of the load-handling attachment by the distance required to provide such space.
Another type of prior quick-disconnect hook assembly features a hook-mounting member, mounted on the rear surface of a load-handling attachment frame, having a vertically-reciprocating hook whose engagement surface, when locked in its engaged position, is located above the upper extremity of the mounting member, thereby requiring no space between the carriage and attachment frame to accommodate such mounting member. The mounting member provides resistance to forward movement of the attachment relative to the bottom of the load carriage by enclosure of the hook within a surrounding vertical slot in the mounting member. However such hook assembly requires a locking pin which projects forwardly into a closed depression formed in the mounting member. This, together with the face that the mounting member encloses the hook in a surrounding fashion, makes the hook assembly susceptible to the build-up of dirt and other debris which can eventually impede the proper operation of the locking structure and the free vertical reciprocation of the hook.
Moreover, while at least some of the foregoing structures provide some vertical adjustability of the engaged position of the hook, none provides any corresponding vertical adjustability of the hook mounting member. Lack of vertical adjustability of the hook mounting member relative to the frame of the load-handling attachment can cause difficulty in mounting of the attachment because the lower horizontal bar of the lift truck carriage must temporarily be placed in a position lower than normal, relative to the load-handling attachment, during the mounting procedure to enable the top of the carriage to be raised into engagement with the upper, downwardly-facing hooks of the load-handling attachment. Accordingly, the upper extremity of the hook mounting member should be sufficiently low to avoid interfering with the abutment of the lower horizontal bar of the carriage against the rear surface of the attachment frame when such bar is in such temporarily lowered position. Since the distance between the top of a lift truck load carriage and the bottom of the lower horizontal bar is not uniform from truck to truck, adjustability of the vertical position of the mounting member relative to the lift truck attachment frame is needed to ensure the necessary clearance.